I found a paper(.doc) from the University of Liepzig that studied colonial revenues in the British Empire in some detail.
Looking over their data, it seems that nearly all parts of the empire were experiencing increasing revenues clear to the end (well..at least until WWII). However, what you do see at the end is that the revenue per capita actually started to decline.
For example, here are their overall gross revenue numbers (in millions of pounds) and per capita (in pounds):
- 1870-71 - 73, 3.21
- 1910-11 - 185, 4.40
- 1925-26 - 812, 18.01
- 1937-38 - 848, 17.95
When you consider that some of this revenue has to go to actually maintaining the empire itself, and many of the costs of that are per-capita as well, one interpretation of this might be that the amount that was straight profit had at least quit increasing at its former rate.
Or to put it more simply, keeping the Empire up had started to become a bit of a chore.